The present invention generally relates to the field of transmission path delay measurements, and more particularly, to a system of determining the time difference of arrival of a frequency shift keyed (FSK) signal at two separate receivers.
Time-difference-of-arrival (TDOA) measurements find applications in one of two areas. In the first type of application, if the propagation velocity of an FSK signal is known, then inferences can be made regarding the signal source location, assuming known receive site locations. In particular, a single TDOA measurement between two receive sites indicates that the source is on a specific hyperbolic spheroid. TDOA measurements between multiple receive sites allows estimation of source location in three dimensions. In the second type of application, if both source and receive site locations are known, then TDOA measurements can indicate the propagation velocity of the FSK signal. The propagation velocity, in turn, can be used to infer characteristics of the propagation medium.
The common method of measuring TDOA is as follows. At each of two receive sites (1 and 2), the received signal is digitized with an analog-to-digital converter (A/D) synchronized with the other site. A set of sequential samples (a frame) is transmitted from site 1 to site 2. At site 2, corresponding frames are compared, via a cross-correlation technique, to estimate the relative delay between the two frames of data, referred to as an estimate of the TDOA. This method of estimating TDOA requires that an entire frame of data be transmitted from one receive site to another. Delay estimation accuracy increases with longer data frames. So the amount of data to be transmitted may be quite large relative to available communications resources.
A need therefore exists for a system and method for determining TDOA values which does not require transmission of an entire frame of data from one receiver site to another.